Oddly the LOTR films have established a sort of consistency.
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If I recall correctly, Gandalf only uses "magic" against other "magical" foes and physical means against physical foes. I wonder will that be maintained in TH. Even more oddly, I suppose, Saruman seems to abide by the same sportsmanship!

But more generally Im not sure audiences of the film or the text tend to think in practice quite the way you suggest. The "why doesn't he use" question could, after all, be levelled at Gandalf in the text on numerous occasions, but tend not to be.